1. Field of the Invention
The invention relates to a process for producing cellulosic shaped objects by extruding and coagulating a solution of chemical wood pulp in a tertiary amine oxide, and possibly a liquid that does not dissolve wood pulp and is miscible with the tertiary amine oxide, the solution capable of being shaped into objects, the process of producing the shapeable solution employing a first wood pulp with a higher degree of polymerization and a second wood pulp with a lower degree of polymerization.
2. Description of Related Art
EP-A-0 648 808 discloses a shapeable cellulose solution made from wood pulp in the tertiary amine oxide N-methylmorpholine-N-oxide (NMMO) and a liquid that does not dissolve wood pulp and is miscible with the tertiary amine oxide, such as water. The wood pulp used is a mixture of two wood pulps, where the first wood pulp has a degree of polymerization, hereafter referred to as DP, between 500 to 2000 and the second wood pulp a DP of 90% or less of the DP of the first wood pulp and lying in the range between 350 and 900. The mixture ratio of the first wood pulp to the second is from 95:5 to 50:50 by weight.
Cellulosic shaped objects are produced from this cellulose solution, whereby the aim is high speed with a process stability that does not lead to degradation of the mechanical properties of the cellulosic shaped objects.
Solutions of wood pulp in the tertiary amine oxide NMMO are generally not anhydrous. In order to produce a shapeable solution of wood pulp in NMMO and water, the concentrations of the three-phase solution, i.e., wood pulp, NMMO, and water, must be adjusted such that each lies in a predetermined range, the so-called solution region in a ternary diagram (see for example WO094/28212).
EP-A-0 553 070 discloses tertiary amine oxides that can be used for producing shapeable and/or spinnable cellulosic solutions and that permit the production of practically anhydrous cellulose solutions. While solutions of wood pulp in NMMO with a wood pulp concentration of 12% by weight and 77.5% by weight NMMO, for example, have a water content of 10.5% by weight, a solution of cellulose in tertiary amine oxides according to EP-A-0 553 070 can be anhydrous or have a very low water content.
In the production of cellulosic solutions in NMMO and water, the procedure is normally that a pre-mixture, also termed suspension, is first produced, in which water is present to such an excess that the wood pulp is not dissolved in NMMO. Water is removed from this suspension under elevated temperature and reduced pressure, for example in an extruder, thin-film evaporator, or evacuable mixer or masticator until the solution region has been reached. This solution is then shaped into the desired objects, such as fibers, filaments, or hollow fibers, in a spinneret, or extruded to form films or membranes, and routed via an air gap into a precipitation bath for coagulating the cellulose. After coagulation, the shaped objects are washed in baths to remove NMMO and then dried.
Such a process is also used for producing the cellulose solution in accordance with EP-A-0 648 808, although production of the suspension uses not only one wood pulp but rather initially two wood pulps with differing DPs, mixed together, before they are brought into contact with NMMO and water to prepare the suspension.
The cellulose solution disclosed in EP-A-0 648 808 permits the production of cellulosic fibers at high spinning rates, and their mechanical properties such as tensile strength and elongation at break correspond to those of fibers made from a solution of only one wood pulp.
It has been shown, however, that the process employed in EP-A-0 648 808 for producing the cellulose solution, whereby wood pulps with differing DP are mixed initially, can lead to problems when wood pulps are used where one has a very high DP, such as 7000, and does not result in a cellulose solution with good shaping properties.